1. Field of the Invention
The present invention relates to a constitution of a thin film transistor having a high characteristics and reliability.
2. Description of the Related Art
Various devices employing thin film transistors are known. However, since the crystallinity of the silicon film constituting the active layer of a thin film transistor is insufficiently low, a large number of carriers are caused to move by the high electric field which is generated between the drain region and the channel region. This causes a problem of high OFF current. Furthermore, the low crystallinity of the silicon film can effect a low withstand voltage, and this causes serious degradation of the device.
As a means for solving the above problems, JP-B-5-44195 (the term "JP-B-" as used herein signifies "an examined published Japanese patent application") discloses a constitution comprising a plurality of equivalent thin film transistors being connected in a serial arrangement so as to lower the voltage applied to each of the thin film transistors.
However, when a device of the above constitution described in JP-B-5-44195 is manufactured and operated, it has been found that a high voltage is applied locally to the thin film transistor on the drain side. That is, in the constitution above, it has been found that the voltage is not distributed equally to each of the thin film transistors, but that the thin film transistor on the drain side alone is subjected to high voltage.
Moreover, when operated under a high voltage, it has also been found that the breakdown or degradation occurs in sequence from the thin film transistor on the drain side.